alx as [deleted] says it wouldn't be a good idea to change the core files. Though you could create an extension that takes care of making the change.
The code to alter would be in the javascript part of Flarum and requires compilation. If you're not familiar with extensions I wouldn't recommend changing that yourself.
Pasting some custom CSS in the Flarum admin panel is definitely the easiest. Let us know if you need help writing those CSS rules.